A known type of antenna array normally has two or more primary antenna elements arranged alongside one another and one above the other, resulting in a two-dimensional array arrangement. These antenna arrays, which are also known by the expression “smart antennas” are used, for example, in the military field for tracking targets (radar). However, recently, these antennas are also being increasingly used for mobile radio, in particular in the 800 MHz to 1000 MHz, and 1700 MHz to 2200 MHz frequency bands.
The development of new primary antenna element systems has also made it possible to construct dual-polarized antenna arrays, in particular with a polarization alignment of +45° or −45° to the horizontal or vertical.
Irrespective of whether they are fundamentally composed of antenna elements with dual polarization or with only single polarization, antenna arrays such as these may be used for determining the direction of the incoming signal. At the same time, however, the transmission direction can also be varied by appropriate trimming of the phase angle of the transmission signals which are fed into the individual columns, that is to say selective beam forming is carried out.
This alignment of the antenna in different horizontal directions is carried out, for example, by means of a beam forming network. A beam forming network such as this may, for example, be formed from a so-called Butler matrix which, for example, has four inputs and four outputs. The network produces a different, but fixed phase relationship between the antenna elements in the individual dipole rows, depending on which input is connected. An antenna design such as this with a Butler matrix has been disclosed, by way of example, in U.S. Pat. No. 6,351,243.
The antenna array which is known from the US patent cited above has, for example, four columns which run in the vertical direction and lie alongside one another in the horizontal direction, and in each of which four antenna elements or antenna element devices are accommodated one above the other. The four inputs for the antenna elements (which in some cases are also referred to as column inputs in the following text) which are arranged in each column are connected to the four outputs of an upstream Butler matrix. By way of example, the Butler matrix has four inputs. This upstream beam forming network in the form of a Butler matrix produces a different but fixed phase relationship between the antenna elements in the four columns in the normal manner depending on which input is connected, that is to say depending on which of the four inputs the connecting cable is connected to. Four different alignments of the main beam direction, and hence of the main lobe, are thus defined. Thus, in other words, the main beam direction can be set to different angular positions in a horizontal plane. Furthermore, of course, and in principle, the antenna array can also be provided with a down tilt device, in addition to this, to vary the depression angle of the main beam direction, and hence of the main lobe.
However, in principle, there are two major problems with antenna arrays such as these using beam forming networks connected in an appropriate manner upstream, for example in the form of a Butler matrix. On the one hand, the main beam direction can generally be adjusted in the azimuth direction only in predetermined steps, which are governed by the different connections corresponding to the number of inputs. By way of example, in the case of a Butler matrix with four inputs and four outputs, only four different azimuth angles can be set on the antenna array in this way.
Furthermore, a specific problem occurs when a Butler matrix is connected upstream for direction forming, since calibration is very complex in this case. This is because the Butler matrix results in the phase angle not being standard. Furthermore, two or more primary antenna elements of the antenna receive a portion of the signal, irrespective of which input of the Butler matrix is connected.
The exemplary illustrative non-limiting technology herein provides a calibration apparatus for a switchable antenna array, in particular for an antenna array with an upstream beam forming network, for example in the form of a Butler matrix, such that the improved calibration will allow the antenna array to be adjusted in the azimuth direction without any problems, with an even greater number of different angles for the beam direction. The exemplary illustrative non-limiting technology herein also provides an appropriate operating method for operating a corresponding antenna array.
It is surprising that it has now become possible, according to an exemplary illustrative non-limiting implementation, to use a beam forming network which is already known per se, for example in the form of a Butler matrix, to adjust the azimuth direction of the antenna array for further angular alignments in addition independently of the predetermined, for example, four, different inputs (via which the antenna can be set to four different transmission angles in the azimuth direction). According to a non-limiting implementation, this is possible in that at least one input of the beam forming network, for example in the form of the Butler matrix, but preferably at least two inputs of this network, is or are fed with an appropriately trimmed and calibrated phase angle, so that it is possible according to produce intermediate lobes, by way of example. It is thus possible to set the transmission directions of the antenna array to additional intermediate angles as well as the predetermined main angles.
In an exemplary illustrative non-limiting implementation, this is possible by phase trimming in advance for the antenna elements which are fed via the Butler matrix, in order that the individual lobes add in the correct phase when, for example, two inputs are connected.
This is preferably achieved in that it is possible to shift the phases upstream of the inputs of the beam forming network, for example in the form of the Butler matrix, at least with respect to the antenna elements which are arranged in some of the columns of the antenna array, such that the antenna elements which are fed are driven appropriately while at the same time connecting two or more inputs in order to achieve the desired swiveling of the lobe.
In the case of a 4×4 antenna array with four columns and in each case four antenna elements or antenna element groups, the phase angles of all the antenna elements are preferably shifted appropriately at the same time.
The calibration of the phase angle can preferably be carried out by means of phase control elements which are connected upstream of the corresponding inputs of the Butler matrix. Alternatively, this can also be carried out by using upstream additional lines to the Butler matrix, which must be chosen to have a suitable length to produce the desired phase trimming.
It has also been found to be advantageous to place appropriate probes on the antenna array itself, via which appropriate calibration signals can be received, in order to carry out the phase trimming by means of a calibration network.
Finally, still further improvement can also be achieved by the combination network containing lossy components. This is because these components contribute to reducing resonances.
Although the phase angle of the transmission from the input of the individual columns or of the antenna inputs is preferably of the same magnitude, the phase angle (or the group delay time) will, however, in practice differ to a greater or lesser extent from the ideal phase angle, due to tolerances. The ideal phase angle is that at which the phase for all the paths is identical, to be precise with respect to the beam forming as well. The discrepancies, which are to a greater or lesser extent dependent on the tolerance, are produced additively as an offset, or else as a function of frequency as a result of different frequency responses. The exemplary illustrative non-limiting implementation herein proposes here that the discrepancies be measured over all the transmission paths, preferably on the path from the input to the antenna array or beam forming network to the probe output or input to probe outputs, and preferably over the entire operating frequency range (for example during production of the antenna). If coupling devices are used, the transmission paths are preferably measured over the path from the input to the antenna array or beam forming network to the coupling output or coupling outputs. This determined data can then be stored in a data record. This data, which is stored in suitable form, for example in a data record, can then be made available to a transmitting device or to the base station in order then to be taken into account for producing the phase angle of the individual signals electronically. It has been found to be particularly advantageous, for example, to associate this data, or the data record which has been mentioned, with the corresponding data for a serial number of the antenna.